Conventional reflection seismology utilizes surface sources and surface receivers to detect reflections from subsurface acoustic impedance contrasts; i.e., subsurface strata. The image and information obtained utilizing conventional reflection seismology is considered relatively poor due to long travel paths between the sources, reflectors, and receivers.
In vertical seismic profiling (VSP), seismic sources are located at the surface, and sensors (geophones) are suspended in the borehole with a wireline. When the seismic sources are initiated, the received data is recorded and processed for several borehole depths. The VSP can have significant technical limitations, for example, each surface seismic source is costly to apply, adds to the acquisition time, and may cause rig inactivity. Another limitation is that the VSP often causes the generation of multiple ghost images due to energy trapped in the surface layer.
Drilling applications where the downhole geophones are added to a measurement while drilling (MWD) tool string, would have to be decoupled from the drill string, or else they would only be capable of recording the arrival of the relatively large first seismic event. Also, downhole geophones are sensitive to tube wave events traveling up and down the borehole. Thus, to date, vertical seismic profiling has not been successful when used in conjunction with MWD applications.
Acoustic pulses could be generated by piezoelectric crystals and other devices lowered into the borehole by a well logging cable. The cable could supply the devices with continuous electrical power and avoid the problem of making multiple trips into and out of the hole. The principal problem with this technique is that the amount of power that could be transmitted through a logging cable would be low, typically about 200 to 300 watts. The pulses generated in this manner would thus be much lower in energy than those generated by explosives. In addition, very long time periods would be required to store enough energy in the downhole tool to discharge into a high-energy pulse. In either case, very long periods of time would be required at each depth in the borehole to transmit sufficient energy by the piezoelectric crystals to produce acceptable signal-to-noise ratios at the detectors on the surface.
Additionally, explosive charges and large vibrator trucks have been employed as commercial seismic sources for many years, but these techniques have several disadvantages, including the unpredictable characteristics of the explosive sources. In recent years, interest has concentrated on the development of controlled, swept frequency seismic sources that can be used downhole with appropriate receivers positioned either in adjacent holes or on the surface. Early downhole sources suggested designs where the source of the vibratory power, be it pneumatic, hydraulic, electrical, or mechanical, was located on the surface and was somehow transmitted to a downhole actuator.
In all of the above noted applications, the seismic source generates a main pulse and several primary waves which provide sinusoidal signals overlapping of the initial pulse generated primary wave. The resonant secondary waves are difficult to distinguish from a clear main pulse and therefore differentiation of the secondary pulses becomes difficult and analysis of the geological data less precise.